I beg to differ on your blanket statement about positional openings being more difficult than tactical ones. More accurate would be that the easier ones to learn are the ones that follow the general opening principles (i.e. Control the center, don't move the same piece multiple times, etc).
The harder ones are the ones that violate the most principals, like giving up the center (i.e. King's Indian), or moving the same piece a kazillion times and falling behind in develpment (i.e. Alekhine's Defense). The reason for this is that openings of that nature there is absolutely no room for error, whatsoever! In openings that follow the "Opening Principles" that anybody under 1800 should be learning to begin with (those under 1800 have absolutely no business trying to memorize reams and reams of theory and lines. Even if you know sideline d532a6 at move 21 of some King's Indian book, it ain't going to help you as your opponent won't even reach that point of the game at the 1700 level.), there is room for error, making a few 2nd best decisions, and not getting blown right off the board.
Therefore, my vote for simplest is actually a positional opening, and my vote for hardest is highly tactical:
Simplest: Queen's Gambit Declined
Hardest: Modern Benoni
Which opening do you think is the simplest, and which do you think is the most difficult to learn? I'll make a top 10 for both cases.
As a rule of thumb I think positional openings are more challenging to learn than tactical ones but of course there are exceptions, the Sicilian Najdorf for example.
I'll start:
Easiest: Giuoco Piano, Hardest: Grunfeld